The present invention relates to a document reading device for sheet-through type image forming equipment and, more particularly, to a document reading device having a document laying member located at a predetermined reading position and implemented as a glass platen and protecting the glass platen against damage when a document pressing member in the form of a back-up roller is inadvertently let to fall or hits against the glass platen.
An electronic copier, facsimile transceiver, laser printer or similar image forming equipment has a document reading device which is often provided with a sheet-through configuration. Specifically, a sheet-through type document reading device illuminates a document by stationary lamps while transporting the document and reads a reflection from the document by a photoconductive element, CCD array or similar reading device via stationary optics. This type of document reading device is disclosed in Japanese Utility Model Laid-Open Publication No. 126512/1988 and Japanese Patent Laid-Open Publication No. 157670/1989 by way of example. Specifically, a document transported by a transporting member in the form of a roller is brought to a stop at a reading position on a glass platen. While the rear of the document is pressed against the glass platen by a document pressing member implemented as a back-up roller, optics disposed below the glass platen reads an image printed on the document. Generally, the back-up roller is movable away from the glass platen by a substantial angle, i.e., it is journalled to free ends of arms which are in turn rotatably mounted on a shaft. Hence, when the surface of the back-up roller or the upper surface of the glass platen is contaminated, the arms may be rotated about the shaft to move the back-up roller away from the glass platen. Then, the back-up roller or the glass platen is readily accessible for maintenance work such as cleaning or replacement. However, it is likely that the back-up roller moved away from the glass platen is inadvertently let to fall onto the glass platen during or after maintenance to break the glass platen and/or to damage the back-up roller. Further, when the document being transported through the document reading device is relatively thick, the back-up roller drops onto the glass platen over a distance corresponding to the thickness of the document as soon as the trailing edge of the document leaves the back-up roller. This is also apt to crack the glass platen.